The present invention relates to a method of recovering dicarboxylic aromatic acids from manufactured articles for recycling made of or containing aromatic polyester resins, such as beverage bottles, fibres and films, or from waste from the processing of these resins.
In particular, the invention relates to the recovery of terephthalic acid from bottles for recycling which are made of PET, polyethyl naphthenates (PEN) or ethylene terephthalic copolymers in which some of the terephthalic acid units are replaced by units derived from isophthalic acid and/of 2,6-naphthalene dicarboxylic acid.
Methods are known for recovering dicarboxylic aromatic acids, in particular, terephthalic acid and 2,6-naphthalene dicarboxylic acid, from manufactured articles for recycling which are made of aromatic polyester resins, in which the resin of the article, suitably reduced to chips or fragments, is depolymerized by hydrolysis with water at high temperature (200-300xc2x0 C.) and under pressure, or with water added with carboxylic acids such as, for example, acetic acid, used as catalysts of the hydrolysis reaction (U.S. Pat. No. 6,103,930).
Owing to the variable nature of the impurities present in the articles (colourings, contaminants, etc.), and to the different sources from which the articles come, in order to produce terephthalic acid which is usable in the preparation of resins that can produce articles with acceptable colour characteristics, treatments for the purification of the raw terephthalic acid, such as catalytic combustion of the impurities present and subsequent catalytic hydrogenation, are required (U.S. Pat. No. 5,414,113).
It has unexpectedly been found that it is possible to produce dicarboxylic aromatic acids, in particular terephthalic acid, of sufficient purity and with satisfactory colour characteristics, from manufactured articles for recycling which are made of or contain aromatic polyester resins, in particular PET and ethylene terephthalic copolymers containing up to 20% in moles of isophthalic acid and/or 2,6-naphthalene dicarboxylic acid units, by reaction, at temperatures between 160xc2x0 and 250xc2x0 C., of the articles, suitably broken up into chip or fragment form, in solution in a carboxylic acid with 2-6 carbon atoms and with a pKa from 4 to 6, preferably glacial acetic acid, in the presence of catalytic quantities of a strong inorganic or organic acid with a pKa of less than 1, preferably aqueous concentrated hydrochloric acid or concentrated sulphuric acid. It is also possible to operate with anhydrous, gaseous hydrochloric acid (hydrogen chloride) or with fuming sulphuric acid.
Inorganic acids are preferred. The quantity by weight of the acids is generally from 0.5 to 50% by weight relative to the resin, preferably from 1 to 20% by weight.
Water may be present in quantities up to approximately 200% by weight relative to the strong acid.
Hydrochloric acid is preferably used in 20-37% by weight aqueous solution. Sulphuric acid is used concentrated at 96%, or else diluted with water.
The reaction is carried out in an autoclave under autogenous pressure. For example, autoclaves made of stainless steel coated internally with Teflon, or of titanium, may be used.
The reaction temperature is preferably from 180xc2x0 to 230xc2x0 C.
The carboxylic acid with 2-6 carbon atoms, preferably glacial acetic acid or glycolic acid, is used in quantities sufficient to solubilize the resin under the reaction conditions; for example, from 2 to 10 parts of acid per part by weight of resin are used.
The article is broken up to produce chips or fragments, for example having for length and/or width dimensions of from 1 to 2 cm. The articles may be coloured or transparent and may contain contaminants, and the fragments may be produced from mixtures of articles of different colours and different origin.
It is also possible to use articles made of or containing polyester resin mixed with polyamides such as, for example, poly(m.xylilene adipamide) or other polymers, in particular, in the form of multi-layer films or of multi-layer bottles in which at least one layer is made of co(polyalkylene terephthalate) and one layer is made of poly(m.xylilene adipamide), or mixtures thereof with polyalkylene terephthalate.
In the case of articles made of blends of polyesters with polyamides, the polyamide dissolves in the reaction medium and does not therefore interfere with the recovery of the dicarboxylic aromatic acid which is present as solid phase.
The reaction is preferably carried out in the absence of oxygen; this is to prevent undesired oxidation reactions.
Upon completion of the reaction (the time is generally a few hours), the autoclave is cooled to bring it to atmospheric pressure. The dicarboxylic acid is separated from the liquid phase by conventional methods, for example, by filtration.
The crude acid may be purified by hot treatment with glacial acetic acid, for example, operating at the reaction temperature.
When terephthalic acid is produced with the use of aqueous hydrochloric acid as the catalyst, a product with a purity of more than 98%, after purification, is obtained.
When the acid is produced with the use of sulphuric acid as the catalyst, the crude acid already has a purity of 97-98%.
In this case, the acid has a grey colouration due to carbon impurities dispersed uniformly in the body of the acid.
Terephthalic acid coloured in this way is suitable for the preparation of resins for bottles by injection blow-moulding, since the parisons produced using this resin dissipate heat easily, thus facilitating the bottle-production cycle.
The grey coloured bottles thus produced are commercially acceptable.
The L colour parameters of the crude terephthalic acid produced with the use of sulphuric acid as the catalyst are between 20 and 80.
The aromatic dicarboxylic acids obtainable according to the process of the present invention, particular reference is made to terephthalic acid, are free from metal compounds deriving from the polycondensation and/or transesterification catalyst used for preparing the polyester resins, in particular are free from antimony compounds.
The method according to the present invention may be performed continuously by recycling of the carboxylic acid after it has been made up with fresh acid and recovery of the acid removed by distillation.
Using acetic acid in for the recovery of terephthalic acid from PET, ethylene glycol monoacetate and diacetate are formed as by-products and are recovered.
The following examples are provided by way of non-limiting illustration of the invention.